1. Field of the Invention
This invention relates generally to a demand assigned multiple access protocol for use in a satellite communications system and, more particularly, to a distributed control demand assigned multiple access protocol for use in connection with a processing satellite communications system to allow multiple users to efficiently use a common uplink transmission resource (TR).
2. Discussion of the Related Art
Various communication systems make use of a processing satellite orbiting the Earth in a geosynchronous orbit. As is known in the art, processing satellites provide the transfer of data between user terminals at ground stations on the Earth. Data is transmitted to the satellite from the user terminals on a satellite uplink. The uplink signals are transmitted on channel slots that are separated in frequency and time. The data that is transmitted on the channel slots is separated or packaged into separate ATM (asynchronous transfer mode) data transfer cells. The ATM cells are demodulated, decoded and switched at the processing satellite, and then transmitted back to the Earth on a satellite downlink. A suitable antenna system is provided on the satellite to receive the uplink signals and transmit the downlink signals over a predetermined coverage area on the Earth.
A common problem for processing communications satellite system is the need to regulate access to the satellite for a multitude of bursty users, for example, a set of intra-corporate inter-site local area network (LAN) interconnections, who mutually share the uplink. In other words, some form of protocol is required to allow the many users to gain access to the satellite on the uplink without interfering with each other. Typically, bursty users require a high peak transmission rate to minimize response time, but have a low duty cycle. If a dedicated uplink TR were allocated to each user at the peak rate, then the average usage of the TR would be low because of the low duty cycle. A TR is defined to be a group of channel slots available for transmitting data on a transmission uplink path at a certain bandwidth.
To solve the above-described problem, demand assigned multiple access (DAMA) protocols have been designed to improve the usage efficiency of the available TRs by allowing several users to share the same uplink TR. In other words, to provide a more efficient use of the available TRs, multiple users share the same uplink bandwidth. In general, DAMA protocols involve trading data throughput efficiency against delay. One of the first DAMA protocols is generally known as xe2x80x9cSlotted Alohaxe2x80x9d, where the multiple users of the common TR compete for resource usage. In the past, Slotted Aloha has been used mainly on so-called xe2x80x9cbent-pipexe2x80x9d satellites where the downlink is simply a frequency and power shift of the uplink and where collision detection is performed by the terrestrial users. Nevertheless, the concept applies equally to processing satellites where the uplink and downlink transmission formats may differ markedly as a result of processing (where the uplink signal is demodulated, decoded, et cetera, the resultant data content switched to the appropriate downlink and then recoded, remodulated, et cetera) except that collision detection typically also involves the processing satellite. In the Slotted Aloha technique, any of the several users of the same TR would transmit data on the uplink whenever they desired. If another user of the same resource also transmitted data at the same time, the two uplink signals would collide at the satellite, resulting in a downlink signal that was unusable. If the user that transmitted the data received the downlink signal in this unusable format, it would know that there was a collision with another user, and would then retransmit the data at a later time. In order to prevent subsequent immediate collisions by the same two users, the delay of the retransmission for each user was randomized. Although, the Slotted Aloha method does increase the usage efficiency of the TRs by statistically multiplexing the resources, there is a significant potential for collisions when multiple users simultaneously access the uplink, necessitating retransmission and concomitant delay by all of the users.
To limit the number of collisions, improved DAMA techniques were devised, generally referred to as reservation DAMAs. The article, Jacobs, Irwin Mark, et al. xe2x80x9cGeneral Purpose Packet Satellite Networksxe2x80x9d Proceedings of the IEEE, Vol. 66, No. Nov. 11, 1978, pp. 1448-1467, describe approaches to reservation DAMA protocols. In a reservation DAMA protocol, the uplink traffic is separated into a main flow where coordinated non-contentious access to the uplink resource is provided through a predetermined reservation procedure, and a control flow which uses the contentious Slotted Aloha technique, but only for reservation requests. The reservation procedure is defined by specifically reserving a particular slot in the uplink TR prior to actually transmitting the desired data. By reserving particular channel slots for transmitting data on the TR, the chance of a collision between users is removed. Collisions are therefore limited to the lower rate control flow resulting in a much higher usable throughput for the main flow of data.
For processing satellites, there can be three general variations of the reservation DAMA technique, depending on where the reservations are controlled. Particularly, the reservations can be controlled at a central terrestrial location, often referred to as a Network Operations Center (NOC), on board the processing satellite itself, or distributed among the set of users using the TR which are regulated by the DAMA protocol. The first approach is useful for bent-pipe and processing satellites, and is currently in use in very small aperture terminal (VSAT) networks. However, this approach has the disadvantages that a heavy processing load to control the DAMA protocol may occur at the NOC, and two trips through the satellite are required for each reservation request, thus resulting in extra delay and increased usage of the TR. The second approach offers ease of synchronization of a new user to the TR, but has the drawback that it requires extra processing resources on the satellite.
What is needed is an uplink TR allocation technique that allows multiple bursty users to efficiently use a common uplink transmission source in an efficient manner that avoids collisions between the users, and maximizes the utilization of system resources. It is therefore an object of the present invention to provide such a technique.
In accordance with the teachings of the present invention, a distributed demand assigned multiple access control scheme is disclosed for use in connection with a processing satellite communications system, where multiple users are assigned to a common TR as part of a sharing set. The DAMA control scheme of the invention makes use of a reservation procedure that includes transmitting set addressed reservation messages in a contentious control flow to reserve data transfer time in a non-contentious main flow. Each user of the sharing set is synchronized with each other and with a known synchronization variable defined by the next available uplink time slot available to transmit data in the main flow. When a user of the sharing set wishes to send data in the main flow, it transmits a set addressed reservation message in the control flow. If the set addressed reservation message is returned in the satellite downlink, the terminal may assume that all of the users of the shared set received the message. The requesting user can compute the reserved time interval in the main flow and that time interval is now owned by that user, and each user in the shared set updates its next available uplink time slot based on this reserved time.
By gaining access to the shared set, each user is limited to a predetermined peak cell rate, sustained cell rate, and a maximum reservation length. The maximum reservation length identifies the largest number of data units transmitted per reservation, and defines a worst case delay that the user must wait to gain a reservation. A user may xe2x80x9cpiggybackxe2x80x9d a reservation by sending a set addressed reservation message during transmission of data in the main flow.
Additional objects, advantages, and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with accompanying drawings.